CC chemokine receptor 3 (CCR3) is a seven-transmembrane G protein-coupled receptor, which binds to a variety of CC chemokines, including eotaxin (CCL11), eotaxin-2 (CCL24), eotaxin-3 (CCL26), MCP-3 (CCL7), MCP-4 (CCL13), and RANTES (CCL5). CCR3 is known to be a major chemokine receptor expressed on allergic inflammatory cells, including eosinophils, basophils, mast cells, and T helper 2-type CD4+ cells (Combadiere et al., J. Biol. Chem. 1995, 270, 16491-16494; Post et al., J. Immunol. 1995, 155, 5299-5305). Eosinophils have been implicated in the pathogenesis of a number of allergic diseases, such as bronchial asthma (Durham and Kay, Clin. Allergy 1985, 15, 411-418; Kroegel et al., J. Allergy Clin. Immunol. 1994, 93, 725-734), allergic rhinitis (Durham, Clin. Exp. Allergy 1998, 28 Suppl. 2, 11-16), atopic dermatitis (Leung, J. Allergy Clin. Immunol. 1999, 104, S99-108), and eosinophilic gastroenteritis (Bischoff et al., Am. J. Gastro. 1999, 94, 3521-3529). It has been demonstrated that activated eosinophils release major basic protein (MBP), which blocks inhibitory M2 muscarinic receptors (M2Rs) on nerves, increasing acetylcholine release and potentiating vagally mediated bronchoconstriction (Evans et al., J. Clin. Invest. 1997, 100, 2254-2262).
Numerous reports indicate that CCR3 plays important roles in allergic conditions. For example, it has been reported that, in both atopic and nonatopic asthma patients, there are increases in both mRNA and protein levels of CCR3 and its ligands, eotaxin, eotaxin-2, RANTES, and MCP-4 (Ying et al., J. Immunol. 1999, 99, 6321-6329). It has also been demonstrated that CCR3 gene deletion impairs eosinophil recruitment in an acute model of experimental asthma (Humbles et al., Proc. Natl. Acad. Sci. USA 2002, 99, 1479-1484; Ma et al., J. Clin. Invest. 2002, 109, 621-628; Pope et al., J. Immunol. 2005, 175, 5341-5350; Fulkerson et al., Proc. Natl. Acad. Sci. USA 2006, 103, 16418-16423). Furthermore, studies have shown that CCR3 antagonists, such as anti-CCR3 monoclonal antibodies, block binding of CCR3-ligands to either CCR3 transfectants or eosinophils, thus blocking chemotaxis of eosinophils induced by CC chemokines, such as eotaxin, RANTES, or MCP-3 (Heath et al., J. Clin. Invest. 1997, 99, 178-184; Grimaldi et al., J. Leukocyte Biol. 1999, 65, 846-853; Justice et al., Am. J. Physiol. 2003, 284, L168-L178). Therefore, CCR3 antagonists are potentially useful for the treatment of inflammatory diseases, such as allergic rhinitis and allergic asthma. In addition, CCR3 antagonists are also potentially useful for blocking infection of CCR3 expressing cells by certain microorganisms, such as HIV, as CCR3 is known to be an entry co-receptor for certain microorganisms.